Process for the production of solid products containing radioactive waste material and products obtained by this process



United States It has become necessary, mainly for safety reasons, to put radioactive residues into a form and into locations so that they no longer constitute a risk of contamination.

In the case of solid waste materials, it is usually possible to collect them in receptacles which have a suflicient resistance to corrosion and are stored under the ground or, generally, immersed in the sea.

In the case of liquid or mud-like substances, the first treatment envisaged generally has the effect of obtaining a mud which is as thick as possible, that is to say containing as little liquid as possible (generally water). This treatment usually involves evaporation, absorption of radioactive ions on ion-exchange resins and/ or co-precipitation. A subsequent treatment has the purpose of trans-. forming the mud into a generally solid product in the form of blocks, which may or may not be enclosed in receptacles, and which are stored under the ground or in the sea.

It will be understood that such blocks can also be obtained from solid radioactive waste materials (earths, constructional materials, ash, etc.) for example by coating with cement.

This treatment generally consists in directly passing to the solid state, without any drying step, however, which is not economic, because of the appreciable waste of heat and the necessity for a subsequent agglomeration stage in a moist medium, the treatment also giving rise to a considerable problem as to the trapping of radioactive dust formed during the agglomeration.

This transfer to the solid state can be eifected:

(a) By the addition of cement to the mud, which has the disadvantage of increasing the volume of the product and also its weight (this weight usually being multiplied by a factor of the order of 2.9 and the volume by a factor of the order of 1.4);

(b) By coating the mud with the aid of a plastics material, generally an unsaturated synthetic resin (for example a urea/formaldehyde resin) which is subsequently polymerised; this technique has the disadvantage of having an exorbitant cost and of almost doubling the volume of the mud;

(c) By emulsifying the mud in a medium containing a bitumen and breaking the emulsion, which leaves an aqueous phase and a solid product containing the radioactive waste and the bitumen; a disadvantage of this process is that breakage of the emulsion is usually a very delicate operation to carry out; also, the product obtained still contains too much water. Also, the solid residue obtained is too plastic because of the necessity of using, in this technique, a low softening point bitumen.

The present invention has the object of providing a process for the manufacture of solid products containing radio-active waste materials which overcomes these disadvantages; these solid products are obtained in a form which does not involve any risk of contamination.

With the radioactive wastes having previously been put into the form of aqueous muds, the process consists essentially in that, at a temperature between the ambient temperature and 95 C., the mud is mixed with a fiuidified atent ice bitumen, in the presence of a surface-active or wetting agent which serves to permit coating of the mud and to facilitate separation of water contained therein, the greater part of this water then being eliminated by decantation and/ or by means of a mechanical device and the bituminous mastic thus obtained then being further mixed at a temperature such that it becomes sufficiently fluid and is finally poured in order to obtain, on cooling, solid blocks of suitable plasticity.

After elimination of the larger part of the water, and before the second mixing, the bituminous mastic can be raised to a temperature of approximately 100 C. in order to distil off completely the residual water.

The products obtained according to this process are also included in the present invention in the form of novel industrial products.

The addition of one (or more) wetting agents has the object of facilitating, with agitation, the production of a water/mud solids/bitumen mixture, as a result of its emollient or softening properties. Also, when the agitation is stopped, this addition permits separation of the larger part of the water of the mud, the quantity of water separating being greatly superior to that obtained by the process referred to above.

The surfactant or wetting agent also has the object of improving the adhesion of the bitumen with respect to the particles in suspension in the mud, which facilitates coating of these particles with the bitumen.

Separation of the water can occur, when the agitation ends, at the temperature at which the mixing has taken place. It can also be carried out more rapidly or can be completed by passing through a suitable mechanical device (for example, a mixer).

In general, the total proportion of the wetting agent with respect to the bitumen added preferably lies between 1 and 20%.

The wetting agent can, for example, be mixed with the mud before addition of the bitumen. It can also be incorporated in the bitumen before contacting it with the mud.

The bitumen can be fluidified, for example, by elevation of the temperature or by admixture with a certain proportion of non-polar solvents. In the latter case, the thus-fluidified bitumen contains preferably 6080% bitumen per se, 15-33% of non-polar solvents and 17% of wetting agents.

The quantity of water eliminated without using distillation can comprise of the total water in the mud. It can be taken that, if distillation of the residual water is begun when the bituminous mastic still contains 40% of the water from the mud, the process is no longer economic. It is thus important for the elimination of the water before distillation to be at least 60% of the total water of the mud, which can readily be obtained in any case.

The present invention allows the use of bitumens which have softening points higher than those of the bitumens utilised in prior art processes, which allows final products of suitable plasticity to be obtained. The introduction of the bitumen can take place in a single stage, before separation of the water, or in two stages, a first stage before separation of the Water and a second stage after distilling off the residual water. In the latter case, the bitumen added after distilling off the water preferably has a softening point which is higher than that added before separation of the water. A product is then obtained which has a suitable plasticity and is resistant to shock.

In the case of a single bitumen, one is advantageously chosen which has a softening point of 40 to 65 C., the temperature at which the first mixing takes place then being from 60 to 90 C. If all the water is eliminated, the residual water is distilled off in the vicinity of 100 C. and, during the second mixing, the temperature rises to 100 to 160 C. If total elimination of the water by distillation is not effected, the temperature during the second mixing is maintained at 60 to 90 C. A final product is then obtained which contains about 5-10% of water. Depending upon the storage conditions envisaged, the products can ensure the desired retention of the radio elements, despite its water content. In certain cases, this allows simplification of the process and apparatus and thus also ensures a more rapid treatment of the radioactive mud.

In the case where two different bitumens are employed, a bitumen is preferably used before separation of the water which has a lower softening point than that in the preceding case, for example of 3045 C. This bitumen is fluidified by addition of a non-polar solvent, which allows mixing to occur at the ambient temperature. A second bitumen, preferably having a softening point of 80130 C., is added during the second mixing stage after elimination of the residual Water by distillation, the temperature then being 120160 C.

The coating obtained according to the process of the invention ensures, in the best conditions, retention of the radio-elements in the final product and perfect cohesion of this product. Economic and total elimination of the water can, if desired, readily be obtained. In all cases, as the various starting materials are inexpensive, the process of the invention is thus very economic.

The bituminous mastic obtained after completion of the coating step is poured hot or cast in order to obtain on cooling a final product in the form of blocks; this operation can be effected in receptacles which can be the subsequent storage receptacles. These blocks can also be stored, after removal from the moulds, without protective receptacles or introduced into other storage receptacles. Thus, this mastic can be poured in any suitable form and, if desired, can be subjected to an agglomeration operation, for example, formed into briquettes.

The total quantities of bitumen added are variable and depend upon the initial proportion of water in the mud. These quantities generally correspond however to a weight proportion of 2842 in the product, which corresponds to a weight proportion of bitumen with respect to the dry mud between 40 and 70%. The ratio weight of final product/Weight of mud treated can thusbe of the order of 0.38-0.75, while the ratio volume of final product/volume of mud treated can be between 0.35 and 0.60 (for mud containing initially 50-75% of water).

There are described below, by way of illustration only, two embodiments of the process of manufacture of radioactive products according to the invention. The features which are described in relation to these embodiments should be considered as forming part of the invention, it being understood that all equivalent features can equally well be used without exceeding the scope thereof.

Example I 100 kg. of a radioactive mud, containing about 60% of water and having an activity of the order of 4 curies per metric ton, were introduced into a double-jacketed mixer heated by circulation of a hot fiuid. A special surface-active or wetting agent soluble in the water constituting the mud was then added, comprising potassium naphthenate. The amount of Wetting agent depended upon the quantity of bitumen which was to be added later, namely 15% by weight with respect to the bitumen.

18 kg. of a bitumen having a softening point between 40 and 60 C. was mixed with the mud. The mixture was heated to a temperature between 60 and 85 C., which caused evaporation of a small part of the water.

The bitumen was introduced at its pumping temperature, namelyl10-l30 C. The quantity of bitumen added was determined as a function of the water content of the mud and its particle size range (by weight, 4.5%, 55%, 28% and 12.5% of the particles had dimensions greater than 34 1., from 12 to 34 from 5.5. to 1 2 and from 1.4 to 5.5,u respectively).

'Exudation of the water contained in the mud occurred rapidly, about 5 to 10 minutes after addition of the bitumen. The clear water exuded readily separated from the bituminous mastic formed and was then removed after decantation. About of the water contained in the mud was thus eliminated. This water had a non-dangerous activity and could be eliminated, after control, in the same manner as an industrial efiiuent water.

The bituminous mastic thus obtained, having a weight of about 64 kg, was subjected to distillation with agitation. All the water still contained in the mastic (about 6.5% by weight of the mastic) was distilled off and condensed. This water was less radioactive than the former (about 1000 times less) and could be put into the drain without danger. The temperature was then taken above C. and mixing was carried out to obtain a suitable viscosity, which was obtained at a temperature of C.

Coating was thus suitably effected and the product (about 60 kg.) was poured hot into special receptacles.

The mixer used for mixing before separation of the water was also employed after distillation of the residual water, so that up to the pouring step, the process can be carried out in the same apparatus. This mixture employs simultaneous contrary kneading, two vertical interfitting supports causing the kneading having different speeds of rotation. Also, a scraper operates against the vertical wall of the casing of the mixture.

The pouring orifice was provided with a hinged cover or with a flow recorder at the base of the casing, which allows total recovery of the final product.

Other characteristics of the bitumen utilised:

Direct distillation bitumen- The ratio of the mass of the product obtained (density 1.80) to the mass of the initial mud (density 1.31) is approximately 0.6 and the corresponding volume ratio is about 0.45.

Example 11 17 kg. of a fluidified bitumen comprising by Weight 70% of bitumen per se, 6% of a wetting agent constituted by calcium naphthenate and 24% of non-polar solvents (for example of the kerosene type) were introduced into a mixer. 100 kg. of radioactive mud containing 50% of water were then introduced into the mixture at a rate of 100 litres/hour. The mixture was slowly kneaded at the ambient temperature. When about half the mud had been added, the clear water exuded from the mud separated from the bituminous mastic being formed. When all the mud had been added, the decanted water was pumped off. The mixer was emptied and separation of the mastic and water was completed by means of a special screw or cylinder system (the total amount of water this eliminated represents 80% of the water contained in the mud).

The bitumen used had the following characteristics: Direct distillation bitumen- Penetration at 25 C. (NFT standard 66.004), 180 to 220 (in tenths of a millimetre);

Softening point (ball and ring method, NFT 66.008),

34 to 43 C.;

Density at 25 C., 1.00 to 1.07;

Maximum loss on heating at 163 C. for five hours (NFT 66.011), 2%;

Minimum percent of residual penetration with re spect to the initial penetration after heating loss test, 70%;

Minimum clarification point in C. (open Cleveland section, NFT 60.118), 23 C.;

Minimum solubility in CS (ASTM), 99.5%;

Minimum ductility at 25 C. in cm., 100.

The bituminous mastic thus obtained had a Weight of about 75 kg. and was forwarded under gravity to a second double-jacketed mixer heated by circulation of a heating fluid. It was subjected to distillation under agitation. All the water in the mastic was distilled off, as well as the major part of the non-polar solvents contained in the fiuidified bitumen.

After passing through a distillation column, these solvents were condensed then separated.

17 kg. of a second hot-pumped bitumen were then added to the viscous mass. Mixing was effected until the desired viscosity was obtained at a temperature of the mass of 130 C. The product obtained (about 75 kg.) was then poured into special receptacles provided for this purpose. The use of storage receptacles by casting or agglomeration in the form of briquettes can be dispensed with if desired.

The mixers used can be of the same type as those referred to in Example I.

The second bitumen has the following characteristics:

Direct distillation bitumen or blown bitumen (bitumen oxidised by means of air)- Penetration at 25 C. (NFT standard 66.004), 2 to 20 (in tenths of a millimetre);

Softening point (ball and ring method, NFT 66.008),

Density at 25 C., 1.03-1.09;

Maximum loss on heating at 163 C. for 5 hours (NFT 66.011), 0.2 to 0.5%;

Minimum clarification point in C. (open Cleveland section, NFT 60.118), 250300 C.;

Minimum solubility in CS (ASTM), 99.5%;

Minimum ductility at 25 C. in cm., 0 to 4;

Average pumping temperature in C., 150-195 C.

The ratio of the mass of the product obtained (density 1.47) to the mass of the initial mud (density 1.58) was about 0.8 and the corresponding volume ratio was about 0.7.

What we claim is:

1. A process for the production of solid products containing radioactive waste material, which comprises mixing an aqueous radioactive mud with a fluidified bitumen in the presence of a wetting agent for a time sufiicient to eliminate a major proportion of the water and so form a bituminous mastic containing the mud solids in a coated form.

2. A process for the production of solid products of low activity containing radio-elements present in radioactive waste material, previously put into the form of radioactive muds, by the addition of bitumen to the mud, such process comprising the steps of admixing a fiuidified bitumen with the mud in the presence of a surface-active agent at a [temperature between the ambient temperature and C. to permit coating of the mud and to facilitate separation of water contained in the mud, eliminating the major part of such water and working the bituminous mastic thus obtained at a temperature such that it becomes sufficiently fluid and is finally poured in order to obtain, on cooling, solid blocks of suitable plasticity.

3. A process according to claim 1 in which after elimination of the major proportion of the water and before the second mixing the bituminous mastic is heated to a temperature of about C. in order to distil off completely the residual Water.

4. A process according to claim 1 in which the wetting agent comprises '1 to 20% by weight of the bitumen.

5. A process according to claim 1 in which the wetting agent is mixed with the mud before mixing the mud with the bitumen.

6. A process according to claim 1 in which the wetting agent is incorporated in the bitumen before mixing the mud with the bitumen.

7. A process according to claim 1 in which the bitumen is fluidified by elevation of its temperature.

8. A process according to claim 1 in which the bitumen is fluidified by addition of non-polar solvents.

9. A process according to claim 8, in which the propor tions by weight of non-polar solvent and wetting agent with respect to the total mass of the bitumen after such additions are respectively 15-33% and l-7%.

10. A process according to claim 1 in which the bitumen is a direct distilled bitumen with a softening point of 40 to 65 C., the temperature before separation of the water being 60 to 90 C.

11. A process according to claim 10, in which after total elimination of the water by distillation the temperature is raised to to 160 C.

12. A process according to claim 1 in which a second bitumen of a different kind is incorporated in and mixed with the bituminous mastic obtained after total elimination of the water.

13. A process according to claim 12, in which the first bitumen is a direct distillation bitumen with a softening point of 30 to 45 C., mixing before separation of the water takes place at the ambient temperature, the second bitumen being selected from the group consisting of a direct distillation bitumen and a blown bitumen having a softening point of 80 to C. and the temperature during mixing of the second bitumen is 120 to C.

14. A process according to claim 1 in which the proportion of water eliminated by mixing is from 60 to 90%.

15. A process according to claim 1 in which the total quantity of bitumen added corresponds to -a weight content of bitumen in the final product of from 28 to 42% References Cited in the file of this patent Anderson et al.: Nuclear Science and Engineering, vol. 12, pp. 106-10 (1962). 

1. A PROCESS FOR THE PRODUCTION OF SOLID PRODUCTS CONTAINING RADIOACTIVE WASTE MATERIAL, WHICH COMPRISES MIXING AN AQUEOUS RADIOACTIVE MUD WITH A FLUIDIFIED BITUMEN IN THE PRESENCE OF A WETTING AGENT FOR A TIME SUFFICENT TO ELIMINATE A MAJOR PROPORTION OF THE WATER AND SO FORM A BITUMINOUS MASTIC CONTAINING THE MUD SOLIDS IN A COATED FORM. 